Multilayered boron nitride/silicon nitride fiber coatings

ABSTRACT

A composite is described which has particular utility in the formation of components for gas turbine engines. The composite broadly comprises a substrate having a surface and at least one layer of a BN/Si 3 N 4  coating on the substrate surface. The coating preferably is formed by alternative layers of a BN material and a Si 3 N 4  material. The substrate may be a cloth material with fibers, such as SiC fibers, woven therein.

STATEMENT OF GOVERNMENT INTEREST

The Government of the United States of America may have rights in thepresent invention as a result of Contract No. NAS3-26385 awarded by theNational Aeronautics and Space Administration.

BACKGROUND OF THE INVENTION

The present invention relates to multilayered boron nitride/siliconnitride (BN/Si₃N₄) coatings and to a method and a system for formingsuch coatings.

Composite coatings are used in a number of gas turbine engineapplications. It is important for these coatings to demonstrateresistance to recession and high temperature oxidation stability.

One of the problems with ceramic matrix composites is the instability ofthe fiber matrix interface to oxidation. The use of boron nitride as areplacement for carbon has showed an improvement in resistance tohigh-temperature oxidation. However, it is common knowledge that theoxidation resistance of CVD boron nitride fiber coatings in the presenceof moisture is considerably lower. CVD boron nitride fiber coatings havebeen known to degrade at room temperature in the presence of moisture.

There has been an investigation on the effect of alternating layers ofSiC and carbon in SiC/SiC composites. It has been found that in such asystem the room temperature mechanical properties are similar to aSiC/SiC composite fabricated with a normal carbon interface. However,when the composite is prestrained beyond the proportional limit (so thatthe matrix was micro-cracked) and subjected to high temperatureoxidation, the composite exhibits brittle mechanical properties. Thishas been attributed to the removal of the carbon interface by oxidation.In order to enable this type of system to work in elevated temperatureapplications, a material with a greater oxidation resistance than carbonhas to be substituted.

Some speculated that boron nitride could be substituted for the carbon.The major problem with such a substitution however is in the processingof the material. Boron nitride is usually fabricated by reacting a boronhalogen with ammonia. The deposition of SiC is affected by the presenceof ammonia. Therefore, the deposition of boron nitride has to be carriedout in a separate reactor. This results in extra heating/cooling cyclesin the process and additional handling of the part from one reactor tothe other. This makes a boron nitride/silicon carbide multilayeringconcept unattractive.

SUMMARY OF THE INVENTION

Coatings provided by the present invention exhibit improved hightemperature oxidation stability.

In accordance with the present invention, there is provided a compositewhich broadly comprises a substrate having a surface and at least onelayer of a BN/Si₃N₄ coating on the substrate surface. The coatingpreferably is formed by alternative layers of a BN material and a Si₃N₄material. In a preferred embodiment of the present invention, thesubstrate is a cloth material with fibers, such as SiC fibers, woventherein.

Further, in accordance with the present invention, a method for forminga coating having high temperature oxidation stability broadly comprisesthe steps of placing a substrate in a reaction chamber, heating thesubstrate to a deposition temperature, and forming at least one coatinglayer on the substrate. The forming step comprises introducing ammoniaand nitrogen into the reaction chamber, introducing a boron halogenprecursor into the reaction chamber, stopping the flow of the boronhalogen precursor, and introducing a silicon halogen precursor into thereaction chamber.

Still further, in accordance with the present invention, a system forforming a coating having a high temperature oxidation stability broadlycomprises a reaction chamber for holding the substrate to be coated andmeans for forming a multilayered BN/Si₃N₄ coating on the substrate.

Other details of the multilayered boron nitride/silicon nitride fibercoatings of the present invention, as well as other objects andadvantages attendant thereto, are set forth in the following detaileddescription and the accompanying drawings, wherein like referencenumerals depict like elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a system for depositing the coatings of the presentinvention;

FIG. 2 illustrates a reaction chamber used in the system of FIG. 1;

FIG. 3 is a TEM thin foil micrograph of a multilayered BN/Si₃N₄ coatingin a SYLRAMIC™ SiC melt infiltrated composite; and

FIG. 4 is a graph showing coating recession distance as a function ofenvironmental exposure for various interface candidates.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The present invention is directed to an improvement in the hightemperature oxidation stability of a boron nitride (BN) interface thatmay be used in ceramic matrix composites (CMC's). CMC's possessing a BNinterface are candidate materials for use in various components or partsfor engines, such as gas turbine engines.

In accordance with the present invention, a coating is provided whichhas an alternating multilayered interface formed from BN/Si₃N₄. Thecoating surrounds fibers, such as SiC fibers in a substrate material,and bonds the fibers to a surrounding matrix. The coating on fibersbecomes the fiber interface in a matrix. This provides the correctamount of de-bonding in the composite. If the matrix is bonded toostrongly to the fibers, the composite acts like a monolithic material.If the fiber interface is too weak, the load is not transferred from thematrix to the fibers. This is the opposite of organic polymer matrixcomposites. There you want a strong bond to the fibers. The use of Si₃N₄is preferred from a mechanical and oxidation standpoint as well as froma processing perspective. Silicon nitride possesses a lower modulus,superior thermal shock resistance, and a higher resistance to oxidationthan SiC. These properties make silicon nitride a preferred material foruse in a multilayered interface coating. The fabrication of siliconnitride is also performed in an ammonia atmosphere. This makes theprocess compatible with the fabrication of boron nitride and enables theprocess to be done in the same reactor.

Multilayered BN/Si₃N₄ coatings, in accordance with the presentinvention, may be deposited on any desired substrate. The substrate maybe a metallic material or a non-metallic materials such as a ceramic.The substrate may also be a fabric cloth material having fibers, such asSiC fibers, woven therein. The substrate may be a preform shaped to aparticular configuration or may be a preform to be cut or shaped lateron. For example, the substrate may be a perform made from commercialmaterials such as those sold under the SYLRAMIC trade name and/or thosesold under the HIGH-NICALON trade name. If desired, the substrate maycomprise one or more layers or plies of a desired material. For example,the substrate may be performs fabricated by laying up eight plies of a5-harness satin weave of either SYLRAMIC or HIGH-NICALON cloth in a[(0°/90°)₂]_(s) lay-up and compressing to a thickness of about 0.60inches (1.524 cm.) in graphite tooling. Any high-temperature fibers canbe used.

If desired, the substrate may be infiltrated with SiC prior to theapplication of the coating. The SiC infiltration may be carried outusing any suitable technique known in the art.

Referring now to FIG. 1, there is shown a system 10 used to deposit thecoatings of the present invention. The system 10 includes a CVD reactor12 having a reaction chamber 15 (see FIG. 2) into which the substrate 17to be coated is placed. The reactor 12 communicates with a source 26 ofa boron halogen gas via line 40, a source 32 of a silicon halogen gasvia line 42, a source 28 of ammonia gas via line 44, and a source 30 ofnitrogen gas via line 46. To control the flow of a respective gas, eachof the lines 40, 42, 44, and 46 is provided with gas actuated on/offvalve 14 and a mass flow controller 16. The system 10 further has athrottling valve 18 for keeping the pressure constant during theprocess, a trap 20 for receiving gaseous by-products of the presentinvention, a vacuum pump 22 for creating a particular pressure in thereaction chamber 15, and a scrubber 24. The scrubber 24 may be flowingwater to scrub the exhaust.

In order to prevent the halogen precursors from reacting with theammonia prematurely, the gases may be introduced separately using a tubeinside tube configuration such as that shown in FIG. 2. The tube insidetube configuration includes a tube 102 having an inlet 104 for thehalogen precursors surrounded by a tube 106 having an inlet 108 for theammonia. The inlet 104 communicates with the halogen precursors and thenitrogen source via line 48. The inlet 108 communicates with the ammoniasource via line 50.

A substrate 17, such as a preform formed from multiple plies of a clothwith woven SiC fibers, is positioned within the reaction chamber 15.Thermocouples 114 are provided to control and measure the temperature inthe reaction chamber 15.

The reaction chamber 15 comprises a quartz containment tube 116 whichsurrounds a layer of insulation 118 which in turn surrounds a graphitesusceptor 120. One or more induction heating coils 122 surround thereaction chamber 15 to provide the necessary heat for the reaction.

The substrate 17, which may be a tooled perform, ceramic cloth, or apart for a gas turbine engine, is placed in the reaction chamber 15 andis preferably brought up to temperature in an atmosphere of flowingnitrogen. The nitrogen enters the reaction chamber 15 via tube 102.Thereafter, a boron halogen precursor is allowed to flow into thereaction chamber 15 also via tube 102. The boron halogen precursor maybe selected from the group consisting of BF₃, BCl₃, and mixturesthereof. The boron halogen precursor is allowed to flow for a timesufficient to form a boron nitride layer having a desired thickness,which time depends on the flow rate size of the reactor. The flow of theboron halogen precursor is then turned off using the valve 14. The pump22 is allowed to pump the reaction chamber 15 for a period of time, suchas 10 minutes, with the ammonia from source 28 and nitrogen from source30 still flowing into the chamber 15. After this time period haselapsed, a silicon halogen precursor is allowed to flow from the tank 32to the reaction chamber 15 for a time period sufficient to form asilicon nitride layer having a desired thickness. The silicon halogenprecursor may be selected from the group consisting of SiCl₄, SiF₄, andmixtures thereof.

In order to form a coating having multiple layers, the foregoing processsteps are repeated. Preferably the coating is provided with six to eightlayers of boron nitride and six to eight layers of silicon nitride.

The deposition rate for the boron nitride formed from fluorideprecursors has been found to be too low if the temperature in thereaction chamber 15 is below 1100° C. Thus, deposition temperatures maybe varied for a BF₃ and SiF₄ system from 1100-1200° C. (2012-2175° F.).Temperatures above 1200° C. produce coated substrates with a thickerdeposit on the exterior plies than on the interior plies. At 1100° C.,the difference in the total coating thickness between the exterior plyand the innermost ply has been found to be less than 0.02 μm. At 1200°C., this difference has been found to be about 0.07 μm. For a BCl₃ and aSiCl₄ system, the deposition temperatures may be kept constant at atemperature in the range of 800° C. to 950° C., preferably at atemperature of 850° C. (1562° F.). Above 950° C., the formation of anundesirable homogeneous nucleation reaction has been observed in theboron nitride deposition. Deposition at a temperature less than 800° C.has shown large amounts of chloride in the coating. A depositiontemperature of about 850° C. produces coatings that were consistent inthickness between the innermost and outermost plies.

It has been found that pressure also has an effect on deposition rate.Therefore, it is desirable to keep the pressure in the reaction chamber15 at a pressure of 5 Torr or less. Preferably, the pressure in thereaction chamber 15 is kept constant at a pressure in the range of from1 to 3 Torr for both chloride and fluoride variations. Most preferably,the pressure is about 2 Torr.

Table I presents the deposition conditions which may be used to deposita multilayered coating with a boron nitride thickness of 0.05 μm and asilicon nitride layer thickness of 0.05 μm.

TABLE I Deposition Flow rates (cm/min) Time Temperature Rate* MaterialBCl₃ BF₃ SiCl₄ SiF₄ NH₃ N₂ (min) ° C. (u/hr) Si₃N4* — — 100 — 400 400 70850 0.037 BN* 75 — — — 400 400 70 850 0.081 Si₃N4** — — — 100 400 400 561100 0.107 BN** — 150 — — 600 600 90 1100 0.066 *Made from the chlorideprecursor. **Made from the fluoride precursor. #Pressure was heldconstant at 2 torr.

For both chloride and fluoride systems, it has been found that themultilayered coatings of the present invention possess an iridescentmulticolor appearance.

The effect of humidity on the strength of melt infiltrated compositeswas performed by subjecting melt infiltrated composites with differentinterfaces to long-term exposure to moisture followed by flexuretesting. The composites were exposed at 140° F. in 95% relative humidityfor 14 days. Four-point bending tests were used to evaluate themechanical performance of the composites both in the as-fabricatedstate, pre-stressed condition, and the post exposure state. Specimenswere pre-stressed to 35 ksi (significantly above the proportional limitstress) prior to environmental exposure to induce matrix cracks andassociate damage within the material. The 35 ksi pre-stress level wasselected based on the average load vs. deflection behavior of thecomposites which indicated that the proportional limit stress rangedfrom 26 to 34 ksi (179 t0 234 MPa). Pre-stressing was accomplished byloading one side of the specimen initially, and then flipping thespecimen over and re-loading on the other side. High temperatureoxidation tests were carried out by placing the composites in a tubefurnace in an atmosphere of flowing air for 500 hours at temperatures of1500° F. (815° C.) and 2000° F. (1093° C.) in an unstressed condition.

In addition, advanced environmental testing was conducted to investigatethe ends-on oxidation rate in the presence of water vapor for fourcomposites with different interfaces. The experimental procedureconsisted of exposing machined composite specimens at temperaturesranging from 1292-1652° F. (700-900° C.) in various oxygen-watermixtures, where the partial pressure of water vapor was either 20% or90%. Exposure time was varied from 1 to 100 hours, and measurements ofthe recession distance of the coating from the exposed ends on themachined surface were made as a function of exposure time and watervapor partial pressure. Measurements of recession distance were made bysectioning the composite specimen in the in-plane direction (parallel tothe plies), polishing the section, and measuring the recession distanceof the coating (by microscope) in several different locations along themachined edge. In general, the recession distances that are reported areaverages of least ten separate measurements.

FIG. 3 is a transmission electron micrograph (TEM) thin foil micrographof a BN/Si₃N₄ multilayered coating in a SYLRAMIC, SiC melt infiltratedcomposite. The fiber appears in the lower right hand corner of themicrograph. The first layer closest to the fiber is boron nitridefollowed by the darker layer, Si₃N₄. The next six layers consist ofalternating BN/Si₃N₄ layers. The matrix surrounding the last Si₃N₄ layeris SiC. Additional TEM thin foil micrographs verify that the coating wasquite uniform throughout the composite. Scanning Auger Microscopyanalysis of the coating confirmed that the individual layers wereconsistent in chemistry corresponding with BN and Si₃N₄.

The results of the mechanical testing are shown in Table II for thefollowing composites: (1) Melt Infiltration (MI) composites with astandard BN interface; (2) a composite having a BN interface producedfrom BF₃ precursor; (3) a composite having a BN interface that wasdeposited slow; (4) a composite having a BN/SiC/BN/SiC multilayeredinterface; and (5) a composite having a BN/Si₃N₄ multilayered coating.All of the composites were first prestrained to 35 ksi beforeenvironmental exposure. The results show that: (1) the test itself didnot process the sensitivity needed to show the effects of moisture onthe strength of the compositions; and (2) that the multilayered BN/Si₃N₄interface composites had strengths in the same range as the baselinematerial. FIG. 4 shows the results of the measurements for the fourinterface development candidates (composites nos. 2-5) as well as thebaseline MI composite (composite no. 1). There are several clear trendsindicated by this data: (1) at constant temperature, higher moisturecontent resulted in considerably greater recession distances; (2) for aconstant moisture content, recession distance was much more severe athigher temperature, and; (3) regardless of the temperature or partialpressure water vapor conditions investigated, the recession rates of theBN/SiC/BN/SiC and BN/Si₃N₄ multilayered interfaces were better than anyof the other composites. The tests showed that the BN/Si₃N₄ multilayeredcoating was by far the best in terms of resistance to coating recession.Even under the most severe conditions of temperature and moisturecontent, the recession distance for this coating was less than 100 μmafter 100 hours. The recession distance of the coating in the baselineMI composite was greater than 2000 μm under the same conditions.

TABLE II Environmental Exposure 1500° F., 2000° F., Type of 500 hrs 500hrs Interface None None* Humidity* in air* in air* Baseline MI 74 55 80— — (1.00) (0.74) (1.08) BN(BF₃), 84 93 91 82 72 1100° C. (1.00) (1.11)(1.08) (0.98) (0.86) Slowest 89 79 86 98 86 Dep. BN (1.00) (0.89) (0.97)(1.10) (0.97) BN/SiC/ 85 70 87 99 95 BN/SiC (1.00) (0.82) (1.02) (1.16)(1.12) BN/Si₃N₄ 83 87 80 82 75 Multilayer (1.00) (1.05) (0.97) (0.99)(0.90) *Specimens were prestressed to 35 ksi on both sides prior toenvironmental exposure Data is formed as follows: top value indicatesresidual strength in ksi bottom value indicates strength afternormalization to as-fabricated strength

It is apparent that there has been provided in accordance with thepresent invention multilayered boron nitride/silicon nitride fibercoatings which fully satisfies the objects, means, and advantages setforth hereinbefore. While the present invention has been described inthe context of specific embodiments thereof, other unforeseeablealternatives, modifications, and variations will become apparent tothose skilled in the art. Accordingly, it is intended to embrace thoseunforeseeable alternatives, modifications, and variations as fall withinthe broad scope of the appended claims.

1-15. (canceled)
 16. A composite comprising: a substrate having asurface; and at least one layer of a BN/Si₃N₄ coating on said substratesurface.
 17. The composite according to claim 16, wherein said coatingcomprises alternating layers of BN and Si₃N₄.
 18. The compositeaccording to claim 16, wherein said substrate is formed from a clothmaterial having fibers woven therein.
 19. The composite according toclaim 18, wherein said fibers are SiC fibers.